The long-term goal will be to obtain a systematic and detailed understanding of the organization and afferent regulation of forebrain cholinergic projection neurons of the rat, especially in areas such as the vertical and horizontal limb of the diagonal band and the sublenticular substantia innominata. In order to achieve this goal, we will identify the sources and chemical characteristics of the afferents the cholinergic projection neurons using a series of combined immunocytochemical and tracer techniques at both the light and electron microscope level. Specific attempts will be made to study the fine synaptic articulation pattern of these afferents on different parts of the cholinergic projection neuron using the recently introduced section-Golgi technique. We also propose to examine biochemically the effect of removal of monoaminergic afferents on the activity of choline acetyltransferase in cholinergic projection neurons. As part of these experiments, we will determine the age-related susceptibility of cholinergic projection neurons to afferent deprivation. Quantitative light and electron microscopic studies of local and extrinsic synapses will also proceed in order to characterize the synaptic input to the cholinergic projection neurons in quantitative terms (synapse/cell ration, cell and synaptic densities). This study will provide important information about the organization of the cholinergic projection system and will test the hypothesis that dysfunction or loss of specific afferents might result in atrophy or degeneration of basal forebrain cholinergic neurons. The integrity of these neurons is apparently necessary for normal cognitive function and atrophy or degeneration of basal forebrain cholinergic neurons is reliably observed in Alzheimer's disease.